Process of coating metal surfaces



Patented Jan. 27, 1942 UNITED STATES PATENTiOFFICE' 2,211,375-cl'fiiiilififiififim l signonto Rust Limited, Montreal, (7

Canada This invention relates broadly to the treatment of metal articlesfor the preparation of the metal surfaces to resist rusting orcorrosion. and to receive surface coatings of various sorts and in somecases for the improvement of characteristics of the metal articles, andthe present application is a continuation in part of my priorapplication,

Ser. No. 36,217, filed August 14, 1935, which issued on June 3, 1941, asU. S. Patent No. 2,244,526.

The primary object of the invention is to provide a process of treatingsurfaces of iron, steel,

ferrous alloys, copper and cuprous alloys, to render them resistant torusting, oxidation and corimparting to metal articles a finish whichdoes not interfere with electric or acetylene welding or brazing andwhich serves to diminish or inhibit electrolytic action betweendifierent metals.

panyofcanada, aeorporationof- Ne Drawing 0 Application .m14,'1oss,,seruu In oaaadad ns 3 19:5 2 Claims. (01.148-0) The treatmentis The molten bath is composed of more than 50% of an alkali metalhydroxide, or mixture of alkali metal hydroxides, and less than 50%of'one or more of the alkali metal nitrates, phosphates and carbonateswith or without bath fluxing or stabllizing agents or both. The additionof a small proportion of manganese dioxide or crystalline I coppersulphate as a flux improves the fluidity Various other objects and theadvantages of the invention may be ascertained from the followingdescription. In its broadest aspect, the invention comprises,

I firstl'y, subjecting the articles to be treated to the action of amolten coating agent comprising a major portion of an alkali metalhydroxide, or

finishing solution. The articles may be washed with water as may bedesired or necessary between the several treatments.

More particularly, the invention comprises the features and combinationsof features herein disclosed together with all such modificationsthereof and substitutions of equivalents therefor as are within thescope of the appended claims.

, ticles immersed will have some influence.

equal surface areas. the casting will require somewhat longer immersionon account of heat loss 1 The invention is primarily intended to providea durable and attractive surface finish on ferrous and cuprous metallicarticles, independently of 1 any other surface coating and which finishis-resistant to rusting, oxidation and corrosion of the metal and formsan improved foundation for othersurfacecoatingk 3 of the bath and thusreduces the loss through dragout on the articles treated. The amount ofeither manganese dioxide or copper sulphate should be between 0.5% and10% of the bath and preferably is about 1% or 2%. Copper sulphateappears also to have a stabilizing effect on the bath, especially athigher temperatures.

If it is desired to impart themost effective rust andcorrosion retardingfinish to the surface of the metal, the bath temperature should bebetween 500 and 900 F. and preferably between 700 and 900 F. The actualtemperature used depends upon the composition of the bath, the metaltreated and the duration of the treatment,

which last depends upon the necessity, or otherwise. of first removingan existing coating.

As the duration of treatment depends to some extent on the temperature,the nature of the arcomparing a compact casting and a thin plate of fromthe bath to the casting.

tardant of rust and corrodon. This coating is so thin it does not causeany significant increase in the dimensions of the articles treatednScrews or bolts with very fine threads are not appreciably tighter inthe cooperating tapped holes after treatment than before treatment. Thinsteel effected most conveniently by immersing the articles to be-treatedin baths ofl Thus,

' The molten bath reacts with the metal to pro- I duce a very stronglyadherent coating practically .integral with the metal, which is anexcellent reand such surface coating applied to articles treatedaccording to this invention are more adherent than when applied to baremetal.

Articles to be treated in the molten bath are merely immersed thereinand no agitation of the articles or of the bath is necessary. Thematerial of the bath is very penetrating and goes into the most minutecrevices. Metal plates closely. packed together receive as good a finishon the contact surfaces as on exposed surfaces and do not show-anytraces of the contactsuch as would occur in electroplating or afterdipping in paint or enamel. Small articles such as bolts, nuts, washersand the like may be treated 'in bulk in baskets.

After immersion in the molten bath, the articles are lifted out, drainedfor a few seconds and then washed with water either by immersion thereinor by spraying. The washing should be very thorough on account of thelarge percentage of alkali metal hydroxide in the molten bath.

The washed articles are next immersed in a neutralizing bath being aweak aqueous solution of hydrochloric, sulphuric, oxalic or other suit-vable acid or an aqueous solution of iron sulphate, maintained preferablyat a temperature of about 160 F., but the temperature may be lower orhigher, for example, from 100 to 200 F. A- l% to 2% solution ofhydrochloric acid will serve while sulphuric or oxalic acids may be usedup to 5%. The preferred bath is iron sulphate, about 4 ounces to eachimperial gallon of water. This solution not only neutralizes any alkaliadhering to the metal but darkens and improves the appearance of thefinish imparted in the molten bath. i

After treatment in the neutralizing bath, the articles may be dischargedand used as they are or may be painted, enamelled or metal plated,

but it is preferred that they be transferred to a finishing bath,preferably after washing to remove any free acid from thesurface'thereof.

This finishing bath is composed of iron sulphate, crystallinehaematoxylin, water soluble nigrosine, tannic acid or other suitablesealing or colouring may be used. The amounts of reagents used may beeither more or less than above stated. This bath is maintainedpreferably at a temperature of approximately 180 F., but the temperaturemay be lower or higher, for example between 100 and 200 F., and thearticles are immersed therein for two to ten minutes and then removedandwashed with water. The resulting finish is more resistant to rust andcorrosion than that produced in the first bath and is a better base forpaint, enamel and metal plating. The finish is of' very good appearance,ranging from gun metal to dark blue or black. If given a coat of oil orclear lacquer, the appearance is similar to good black enamel, Inaddition to colouring the finish The coating- Instead ofcrystallineimparted in the first bath, the materials used enter thepores or intermolecular spaces of the finish and seal the same againstentrance of rust or corrosion producing agents.

The neutralizing bath is very important if a paint, lacquer or enamel isto be applied subsequently as every trace of alkali must be removed toavoid'the appearance of blemishes in the final coating.

The condition of the second and third baths I Fmsr Barn Example I PartsSodium hydroxide 65 Potassium nitrate 45 I Example If 7 Sodium hydroxideto 80 Trisodium phosphate 40 to 20 Example HI Sodium hydroxide Sodiumcarbonate 35 Example IV. Sodium hydroxide 35 Potassium hydroxide 20Sodium nitrate 45 v Example V Sodium hydroxide 35 Potassium hydroxide 35Trisodium phosphate 30 Example VI Sodium hydroxide 60 Potassium nitrate30 Trisodium phosphate 10 Example'VII I Sodium hydroxide 65 Potassiumnitrate 30 Potassium carbonate 5 Example VIII Sodium hydroxide 30Potassium hydroxide 35 Sodium nitrate-.. -1 25 Trisodium phosphate 10Example IX Sodium hydroxide 30 Potassium hydroxide 35 Potassium nitrate20 Sodium carbonate l5 Example X Potassium hydroxide "10 Sodium nitrate30 To any of the foregoing mixtures, there may be added, if desired,from 0.5% to 10% of manganese dioxide or crystalline copper sulphate asa fluxing and stabilizing agent, the

m any of the foregoingexamples, the alkali metal compounds named may bereplaced by the similar compound of the other alkali metal.

' The surface finish imparted according tothis invention, and especiallythat resulting from immersion in all three baths,'is very durable andtenacious, so that metal sheets thus treated may be formed in dieswithout injuring the finish.

The finishing treatment, especially when temperatures between 600 and900 F. areused, has

- an annealing effect which improves the strength 0! the metal. Incertain cases where annealing is required between forming operations,the parts may be treated according to thisinvention between the formingoperations and annealing thus efiected simultaneously with surfacefinishing;

It is possible also to heat the bath above 900 F.,

say to 1200 or even 1500 F., to heat treat articles for removal ofinternal stresses therefrom or for otherpurposes.

If no subsequent surface coating is to be applied, it is preferable touse iron sulphate in the second or neutralizing bath and the thirdbathmay be omitted, but a better appearing ilnish'and one more resistant torust and corrosion is obtained if the articles are treated in the thirdbath also; It is usually preferableto use iron sulphate inone only orthe baths, that is, 11 11-. is used in the second bath Lone oi the otheragents isused in the third bath, but if the sulphate lito be used inthethird bath. the second bath is preferably made with hydrochloric orother acid.

Having thus described my invention. what I claim is:

- l. A process of treating articles 01 iron. copper 2. A process oftreating articles of iron, copper and alloys of each of them, whichprocess comprises fusing tosether manganese dioxideand a mixture ofalkali metal compounds including aikali metal hydroxide, alkali metalcarbonate and alkali metalnitrate, the resulting mass being in themolten state reactive with. the metal treated to produce a surfacecoating thereon, and subjecting the articles to.the action of the molten

